Regulation and light-harvesting complex II association of a Dunaliella protein homologous to early light-induced proteins in higher plants.

Abstract

The unicellular green alga Dunaliella bardawil responds to high light, nutrient deprivation, and several other types of stress by massive accumulation of beta-carotene. We have previously cloned a nuclear gene, cbr, that is co-induced with accelerated carotenogenesis. The predicted product of cbr is closely related to early light-induced proteins (Elips) of higher plants, and also shows resemblance to chlorophyll a/b-binding proteins. In the present study, the determination of the cbr transcription start site supported the previously proposed site of translation initiation. Antibodies raised against a synthetic oligopeptide matching the predicted sequence of Cbr recognized two polypeptides of apparently 17 and 19 kDa that were induced in parallel to cbr transcript accumulation in highly illuminated or sulfate-starved D. bardawil cells. The antibodies also cross-reacted with an approximately 20-kDa polypeptide in sulfate-starved Dunaliella salina. In both D. bardawil and D. salina, Cbr was found exclusively in the thylakoid membranes. After mild solubilization, Cbr co-fractionated with light-harvesting complex II (LHCII) in sucrose gradient centrifugation and gel electrophoresis and was specifically associated with a minor LHCII complex. An occasionally observed, faster mobility Cbr form, free of LHCII, was probably released from the larger complex. These results support the conclusion that Cbr belongs to a class of stress-induced proteins transiently associated with antennae complexes.